Data from the Kaiser-Permanente Birth Defects Study are being used to evaluate differences in birth weight and gestational age among four different ethnic groups. The ethnic groups included in the Kaiser study are Whites, Hispanics, Blacks and Asians. In addition to studies of ethnic differences, we have evaluated the effects of smoking and drinking during pregnancy on preterm births. Univariate and multivariate statistical analyses of the effects of 23 factors on the ethnic differences in birth weight have shown that substantial ethnic differences in birth weight remain after accounting for differences in 23 relevant covariates. Compared to Whites, the relative mean differences in birth weight are estimated as -246 for Blacks, -210 for Asians, and -105 for Hispanics. The low birth weight rates and crude odds ratios are 7.7% (2.17) for Blacks; 5.57% (1.57) for Asians; and 4.00% (1.13) for Hispanics. The study concluded that factors currently used to control for ethnic differences in birth weight are insufficient to explain the observed differences. Similar analyses are being conducted to evaluate the ethnic differences in mean gestational age and preterm births (those less than 37 weeks gestational age). Preliminary results show that the ethnic group specific rates of preterm birth are 6.1% for Whites, 11.8% for Blacks, 8.7% for Hispanics, and 7.9% for Asians. Controlling for possible confounding did not substantially change the odds ratios. The effects of cigarette smoking and of alcohol consumption on length of gestation were also examined in this population.